Nothing returns carbon more dramatically to the atmosphere than burning, whether it's torching logs to make charcoal for fuel (above) or igniting coal to run a power generator. Humans have been cutting and burning trees for millennia, but preindustrial societies never added enough carbon dioxide to the atmosphere to disrupt the global carbon cycle. It's the more recent burning of coal, oil, and natural gasprehistoric carbon depositsthat threatens to spin the cycle out of control. Of the total fossil-fuel carbon dioxide emissions since the rise of industrialized societies 250 years ago, half have come in the past 30 years. The heat-trapping ability of the added carbon dioxide molecules has enhanced the natural greenhouse effect, with heat reflected by the atmosphere returning to Earth to raise its temperature. Unless the world cuts its reliance on coal, oil, and gas for power, most scientists believe that the world climate will continue to change, bringing higher sea levels and more extreme floods and droughts. That's why learning how the carbon cycle works has become a public duty.